The present invention relates to a pixel circuit using a light-emitting element, which emits light having the intensity corresponding to an amount of the current, such as an organic light-emitting diode element, to a light-emitting device, and to an image forming apparatus.
Recently, next-generation light-emitting elements, which are an alternative to liquid crystal elements, such as an organic electroluminescent element and an organic light-emitting diode (hereinafter, simply referred to as ‘OLED element’) which are called as light-emitting polymer elements or the like have been of interest. An image forming apparatus using a line head, in which a plurality of OLED elements is provided in one line, as an exposing unit has been developed. In such a line head, besides the OLED element, a plurality of pixel circuits including transistors for driving the line head is disposed. Therefore, recently, the line head composed of the OLED elements arranged in one line has been proposed (for example, see Japanese Unexamined Patent Application Publication No. 11-274569).
Here, the plurality of pixel circuits are arranged in one direction, to which selection signals are supplied through common wiring lines and data signals are supplied through matrix wiring lines. When the selection signals become active, the data signals are supplied to the pixel circuit.
Meanwhile, in order to decrease the brightness variation of the OLED element due to the on-resistance variation of a driving transistor, the on-resistance of the driving transistor should decrease sufficiently compared to the resistance of the OLED element. The size of the driving transistor should be enlarged to make the on-resistance of the driving transistor small.
However, if a driving force of a driving circuit, which is located at a previous stage to supply a current to a gate of the driving transistor, is insufficient, it is impossible to sufficiently drive the driving transistor. Further, if the driving force is insufficient, it takes a long time to change electric potential of the gate. For this reason, the writing operation cannot be terminated within a predetermined period of time, which degrades the printing quality.